Imagine that you have a 100 W incandescent light bulb , and it is shining unifor

Question

Imagine that you have a 100 W incandescent light bulb , and it is shining uniformly into 4? steradians (outward equally in all directions). Please estimate for me (without a calculator). a) What is the efficiency of an incandescent light bulb? b) what is the power of visible light photons produced? c) what is the rate of production of visible light photons? d) your eye receives ~1000 photons per second What intensity (of photons per second through the pupil) does this correspond to? e) Over what area do the photons from your light have to be spread to provide you with the intensity you calculated above? from how far away in a perfectly dark night could you see a 100 W light bulb?

Explanation / Answer

Incandescent light bulb releases most of the energy as heat. It releases only 1% of the energy as light.

a> efficiency of 100W lamp = 1%.

b> Power of visible light photons produced = (1/100)*100 = 1W.

c> 1W = 1J/s Assuming wavelength to be around 4200m, we get energy of each photon as 3ev. (E = hc/lambda).

So number of photons produced = 1/ ( 3 * 1.6 *10^(-19)) = 2.08 x 10^(18)

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